Thick film dielectric structures provide for superior resistance to dielectric breakdown, as well as a reduced operating voltage, compared to thin film electroluminescent (TFEL) displays e.g. as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure when it is deposited on a ceramic substrate will withstand higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. With these advantages and with recent advances in blue-emitting phosphor materials, displays have approached the luminosity and color coordinates required to achieve the technical performance of traditional cathode ray tube (CRT) displays. Nevertheless, further improvement in blue phosphor performance is required to simplify display design, to improve display reliability by lowering operating voltages and to keep pace with a trend towards higher color temperature specifications for displays.
Cerium-activated strontium sulphide phosphor materials have traditionally been used in electroluminescent displays for blue colors while manganese-activated zinc sulphide have been employed for red and green colors. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary color coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese-activated zinc sulphide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per Watt of input power. Cerium-activated strontium sulphide phosphor has an energy conversion efficiency of 1 lumen per Watt, which is relatively high for blue emission. However, the spectral emission for these phosphors is quite wide, with spectral emission for the zinc sulphide-based phosphor material spanning the color spectrum from green to red and that for the strontium sulphide-based material spanning the range from blue to green. This necessitates the use of the optical filters. The spectral emission of the cerium-activated strontium sulphide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Alternative blue phosphor materials having narrower emission spectra to provide the color coordinates required for a blue sub-pixel have also been developed. These phosphor materials include cerium-activated alkaline earth thiogallate compounds which provide good blue color coordinates, but exhibit relatively poor luminosity and stability. The relatively poor luminosity is in part due to their relatively high dielectric constant and optical index of refraction, which decreases the efficiency with which light generated within the materials can be extracted to provide useful luminance.
Thiogallate and thioaluminate phosphor materials containing oxygen have also been developed. U.S. Pat. No. 5,656,888 discloses a method in which minor concentrations of oxygen are added to improve the luminous efficiency and CIE colour coordinates of alkaline earth thiogallate phosphor materials. In the method, an oxide is added to the deposition source materials or alternatively, oxygen is added to the deposition atmosphere. However, excessive oxygen doping to concentrations higher than 4 atomic percent in Sr0.5Ca0.5Ga2S4:Ce resulted in a loss of luminous efficiency due to the undesirable formation of oxides as a separate crystal phase.
Japanese patent application 2000-081483 discloses a method for the oxidation of a vacuum deposited two-layered europium activated alkaline earth thioaluminate phosphor film. Oxygen is added to the annealing process which is carried out at a temperature in the range of 700° C. to 1000° C. under an atmosphere of argon containing 1 to 20% oxygen for a time of about 2 minutes. The annealing process is carried out after the thioaluminate film is coated with a layer of zinc sulphide of thickness in the range of 1000 to 5000 Angstroms in order that the thioaluminate material is not in direct contact with the annealing atmosphere. The annealed thioaluminate film is reported to fractionate into two layers, one comprising the alkaline earth element of aluminum, sulphur and oxygen and the other comprising aluminum oxide.
The aforementioned layered oxidized phosphors are also described in the Japanese Journal of Applied Physics Vol. 40, 2001, pages 2451–55. Annealing of the phosphor is carried out at a temperature of 920° C. under an argon atmosphere unintentionally doped with oxygen. Oxygen is thought to be inadvertently introduced during film deposition and annealing or from the presence of oxide in the deposition source materials. The x-ray diffraction data for the phosphor film shows the presence of barium thioaluminate, alumina and an amorphous phase that is tentatively identified as amorphous barium aluminate. Within the layered film the XPS data indicates an aluminum to oxygen atomic ratio in the aluminum oxide layer of 2:3, meaning that this layer consists essentially of Al2O3. The ratio of elements in the layer containing barium, aluminum, sulphur and oxygen indicates a composition with an empirical stoichiometry of approximately BaAl2S2.6O1.4 such that the stated average empirical composition of the two layers together is BaAl2O2.1S1.9.
European patent application 1,170,351 A2 discloses a barium aluminum oxide phosphor matrix material doped with sulfur to improve the spectral emission properties. A first phosphor composition of Ba:Al:O:S:Eu is deposited by reactive sputtering in a hydrogen sulphide containing atmosphere followed by annealing at 750° C. in air to introduce oxygen. This composition corresponds to the empirical formula BaAl2.19O7.93 S0.95Eu0.03, which has a very high oxygen to sulphur ratio, with the overall composition similar to a mixture of europium doped Al2O3 and BaSO4. A second composition even richer in oxygen that the first described above is formed by introducing oxygen into the vacuum deposition atmosphere and annealing under vacuum. The phosphor stability is stated to be improved if deposition and annealing conditions are adjusted so that the ratio of sulphur to oxygen plus sulfur is in the range of 0.7 to 0.9 corresponding to an oxygen to sulphur ratio from 0.11 to 0.43. The phosphor compositions disclosed are admittedly matrix materials containing sulphide and oxide in which the function of the oxide in the phosphor is to provide a stable coating on the sulphide component to stabilize it against degradation by exposure to the ambient environment.
European patent application 1,170,350 A2 discloses the use of an electroluminescent phosphor stack containing a layer comprising a matrix material of barium aluminate and a sulphur-bearing compound. The method to produce the aluminate phosphor material is similar to that taught in EP 1,170,351 with an average sulphur to sulphur plus oxygen ratio of 0.7 and 0.9 or between 0.02 and 0.5, the latter range having an oxygen to sulphur ratio between 1 and 50.
The aformentioned oxysulphide phosphor materials are typically matrix materials or layered structures containing an oxide and a sulphide layer, in which any of the oxygen introduced into the materials is done so in an uncontrolled or inadvertent manner. Such an uncontrolled and/or inadvertent addition of oxygen adversely affects the crystal lattice structure of the phosphor material leading to negative effects on luminosity and/or stability of the phosphor material.
In view of the foregoing, there remains a need to develop new phosphors having improved properties that obviate the shortcomings of the prior art and that also have use as thin films in electroluminescent displays. The present invention fulfills this and other needs.